This invention relates to radio receivers. In particular, it relates to a receiver that is adapted for realization in an integrated circuit.
A feature of radio receivers since the general adoption of the superheterodyne receiver has been the use of a local oscillator that is tunable. The local oscillator produces a signal that is mixed with an incoming signal to produce an IF signal at a frequency that is fixed. This is sometimes combined with double or triple conversion so that the receiver has two or three intermediate frequencies. These generally have been fixed, and an advantage of the superheterodyne receiver has been the fact that the tuning is essentially limited to the high-frequency end. This includes a local oscillator and any stages of amplification up to a first mixer. After the first mixer, which may be the only mixer, the principal elements of selectivity are the fixed-tuned IF stages. It has been easier to achieve a relatively high level of selectivity in such stages because it is not necessary to tune them.
The features that have been described as advantages of the superheterodyne receiver can become disadvantages when it is desired to make a radio receiver in the form of an integrated circuit. The selectivity of the IF stages in a typical superheterodyne receiver is achieved by the use of tuned circuits that typically involve parallel LC circuits. Electrical isolation to prevent oscillation is typically achieved in such circuits by placing the tuned circuits in metal shields. The result is to produce a number of relatively high-volume elements that use valuable space and are not well adapted to be placed in integrated circuits.
A further feature of integrated circuits is the relative ease with which additional active elements are included in the design of such a circuit. In general, it is easier to add several transistors to the design than it is to incorporate a single inductor or capacitor. The availability of such transistors in an integrated circuit makes it easier to use active filters which both minimize or eliminate the use of inductors and capacitors and also make it easier to vary certain of the parameters of the filters.